1. Field of the Invention
The present invention relates generally to magnetic sensors used in hard disk drives incorporated in computer systems; and more specifically to a method of testing for magnetic sensor response abnormalities.
2. Description of the Background Art
Magnetic sensors used to read the recorded information previously written on a disk in a disk drive are expected to meet several performance criteria. One of the most important criterion is that the response of the sensor be predictable and free from nonlinear response abnormalities. It is desirable to be able to evaluate the sensors for these abnormalities. The method used for evaluation must be fast, inexpensive, and above all reliable in identifying those sensors which have response abnormalities.
Sensors used today in disk drive applications are either magnetoresistive (MR) or giant magnetoresistive (GMR) sensors. The latter are also known as spin valve sensors. Tunnel junction sensors are also being developed for use in disk drives. All of these sensors are constructed by forming multiple layers of thin films. The sensor output changes in response to the magnitude of an external magnetic field. Within the disk drive the external magnetic field comes from the written transitions on the disk. When testing sensors outside of a disk drive, the external magnetic field is provided by a coil or other means.
In order to be useful, the response of the sensor must be reasonably linear with respect to changes in the applied field. Some of the thin films in the sensor are magnetic and therefore are subject to anomalous magnetic behavior. For example the sensor may exhibit Barkhausen abrupt domain movement in the magnetic films within the sensor. A transfer curve, also called a transfer function, is a plot of sensor output, either voltage or resistance, vs. applied field magnitude. A Barkhausen domain movement can be identified by observing a discontinuity, also called a jump, in a transfer curve. A kink is similar to a jump but is observed as a more gradual discontinuity in the transfer curve. Another type of undesirable sensor behavior is observed as an open loop in the transfer curve. Open loops occur when the films within the sensor are magnetically switched through a hysteresis loop. Open loops are also nonlinear response abnormalities in the transfer curve. Nonlinear response abnormalities can cause errors when detecting magnetic transitions on a disk. The ability of removing the sensors which have response abnormalities from the general population of sensors results in an improvement in the overall quality of the group of sensors which is subsequently placed in disk drives.
It is known to test for the presence of sensor response abnormalities by placing the sensor in a magnetic field and monotonically increasing the magnitude of the field while measuring the resistance. When the resistance of the sensor abruptly changes value over a small difference in applied field magnitude, the sensor has a nonlinear response abnormality and is considered defective. One shortcoming of this method is that some sensors which have response abnormalities are not discovered. While the response abnormality of a large sensor tends to be sharp and well defined, a typical response abnormality of a small sensor is more gradual making detection more difficult. This is a particularly serious shortcoming since future sensors will be much smaller in order to support higher recording densities.
What is needed is a testing method to determine the presence of sensor response abnormalities which is efficient, reliable, and works well with small sensors.
In a preferred embodiment, the invention provides for a testing method which is efficient in detecting sensor response abnormalities. Furthermore one embodiment of the invention is capable of detecting the presence of abnormalities which are not well defined.
In a preferred embodiment of the present invention, the value of the applied magnetic field is incrementally changed and the direction of the applied magnetic field is alternated at each magnetic field value. The detectability of any response abnormality is substantially improved when using an alternating field to excite the sensor.
Another embodiment of the present invention provides an improved method for the efficient detection of subtle and ill-defined response abnormalities by examining the departure of the transfer curve of the sensor from an ideal linear transfer curve. In another embodiment of the present invention, multiple values of the sensor voltage or resistance values are measured at each field value and the range of sensor values are examined to indicate abnormalities.
Other aspects and advantages of the present invention will become apparent from the following detailed description which when taken in conjunction with the accompanying drawings illustrate by way of example the principles of the invention.